1. Field of the Invention
The invention relates to methods and systems for improving the fuel economy achieved by xe2x80x9clean-burnxe2x80x9d engines whose exhaust emission control devices periodically require engine operation at an air-fuel ratio rich of the stoichiometric air-fuel ratio.
2. Background Art
The prior art teaches use of an emission control device for a vehicle powered by a fuel-injected, internal combustion engine, such as a gasoline-powered engine, that xe2x80x9cstorexe2x80x9d a constituent gas of the exhaust gas flowing through the device when the exhaust gas is lean, as when the engine is operated with a ratio of engine intake air to injected fuel greater than the stoichiometric air-fuel ratio. Any such xe2x80x9cstoredxe2x80x9d constituent gas is subsequently xe2x80x9creleasedxe2x80x9d when the air-fuel ratio of the exhaust gas flowing through the device is subsequently made either equal to or rich of the stoichiometric air-fuel ratio, as occurs when the engine is operated with a ratio of engine intake air to injected fuel that is equal to or less than the stoichiometric air-fuel ratio. The prior art teaches the desirability of precisely controlling the time period during which the device stores the constituent gas (the xe2x80x9cfill timexe2x80x9d) and the time period during which stored gas is released from the device (the xe2x80x9cpurge timexe2x80x9d) in order to maximize vehicle fuel efficiency obtained through lean-burn operation while otherwise seeking to minimize vehicle emissions.
Unfortunately, when oxygen-rich exhaust gas initially flows in series through a plurality of emission control devices during xe2x80x9cleanxe2x80x9d engine operation, excess oxygen is often stored in the upstream device. When the exhaust gas is later transitioned from xe2x80x9cleanxe2x80x9d to xe2x80x9crich,xe2x80x9d as when seeking to xe2x80x9cpurgexe2x80x9d the stored constituent gas from the downstream device, the engine must burn a significant quantity of fuel with an air-fuel ratio rich of stoichiometric before HC and CO appears in the exhaust gas flowing out of the upstream device into the downstream device. More specifically, the oxygen previously stored in the upstream device must first be depleted by the excess hydrocarbons found in the rich device-purging air-fuel mixture before the excess hydrocarbons in the air-fuel mixture xe2x80x9cbreak throughxe2x80x9d to the downstream device. This fuel penalty occurs each and every time the engine operating condition transitions from lean operation to rich operation, thereby significantly reducing the fuel savings otherwise associated with repeated lean operation of the engine.
And, as the frequency of device purge events increases due to the correlative decrease in nominal device efficiency due, for example, to the accumulation or xe2x80x9cpoisoningxe2x80x9d of the downstream device with SOX, the fuel penalty associated with upstream device break-through also increases. Moreover, relatively higher vehicle loads may precipitate an increase in the temperature of the upstream device, whereupon the upstream device""s nominal oxygen storage capacity and, hence, the fuel penalty associated with upstream device break-through, will also likely increase.
Further, for vehicles equipped with a pair of upstream emission control devices, as may be found in vehicles having either a xe2x80x9cVxe2x80x9d-configuration engine or an xe2x80x9cIxe2x80x9d-configuration engine with a split exhaust configuration, oxygen is stored in both upstream devices during lean operation. Accordingly, twice the amount of fuel is required upon transitioning from xe2x80x9cleanxe2x80x9d to xe2x80x9crichxe2x80x9d engine operation before excess hydrocarbons (namely, HC and CO) break through the upstream devices for use in purging the stored constituent gas from the downstream device.
The inventors herein have recognized a need to provide a method and system for purifying the exhaust gas of an internal combustion engine which is characterized by a reduced fuel penalty when transitioning from lean to rich in order to effect a purge of a downstream emission control device, particularly for those exhaust systems which employ a pair of upstream emission control devices.
Under the invention, a method is provided for controlling the operation of an internal combustion engine having a plurality of cylinders respectively burning an air-fuel mixture to generate exhaust gas formed of one or more constituent gases, wherein each cylinder being associated with a selected one of exactly two cylinder groups, and wherein the exhaust gas from each cylinder group flows through a respective upstream emission control device and then through a common downstream emission control device, with the downstream device storing an amount of a selected constituent gas, such as NOx, when the exhaust gas flowing through the downstream device is lean of a stoichiometric air-fuel ratio and releasing a previously-stored amount of the selected constituent gas when the exhaust gas flowing through the downstream device is rich of the stoichiometric air-fuel ratio. The method comprises supplying a first air-fuel mixture characterized by a first air-fuel ratio lean of the stoichiometric air-fuel ratio to each cylinder groups, whereby the selected constituent gas is stored in the downstream device; and determining a need for purging the downstream device of a previously-stored amount of the selected constituent gas. Upon determining such a need for purging the downstream device, the method further includes supplying a second air-fuel mixture to the cylinders of the first cylinder group while simultaneously supplying a third air-fuel mixture to the cylinders of the second cylinder group, wherein the second air-fuel mixture is characterized by a second air-fuel ratio at or near the stoichiometric air-fuel ratio (hereinafter a xe2x80x9cnear-stoichiometric air-fuel ratioxe2x80x9d) and the third air-fuel mixture is characterized by a third air-fuel ratio rich of the stoichiometric air-fuel ratio, such that, when the second and third air-fuel mixtures flow together through the device, the second and third air-fuel mixtures combine to form a fourth air-fuel mixture characterized by a fourth air-fuel ratio rich of the stoichiometric air-fuel ratio. In a preferred embodiment, the fourth air-fuel ratio is preferably perhaps about 0.97 times the stoichiometric air-fuel ratio and is preferably no greater than about 0.75.
In a preferred embodiment, the step of determining the need for releasing previously-stored constituent gas from the downstream device includes determining a value representing an estimate of the incremental amount of the selected constituent gas currently being stored in the downstream device; calculating a measure representing the cumulative amount of the selected constituent gas stored in the device during a given lean operation condition based on the incremental stored-NOx value; determining a value representing an instantaneous capacity for the downstream device to store the selected constituent gas; and comparing the cumulative measure to the determined capacity value. In a preferred embodiment, the step of calculating the incremental storage value includes determining values representing the effects of the instantaneous device temperature, the cumulative amount of the selected constituent gas which has already been stored in the device, and an estimate of the amount of sulfur which has accumulated in the device. Similarly, in a preferred embodiment, the step of determining the value for instantaneous device capacity includes determining values representing the instantaneous device temperature and the estimate of accumulated sulfur.
In accordance with another feature of the invention, the method preferably includes matching the torque output of the cylinders of the second cylinder group (operating with a relatively enriched air-fuel mixture) with that of the first cylinder group (operating at near-stoichiometry), as by retarding spark to the cylinders of the second cylinder group when operating those cylinders are operating with the enriched air-fuel mixture. Alternatively, the invention contemplates selecting the second and third air-fuel ratios, respectively, such that the torque generated by the cylinders of the second cylinder group operating with the third (enriched) air-fuel mixture is approximately equal to the torque generated by the cylinders of the first cylinder group operating with the second (near-stoichiometric) air-fuel mixture.
In accordance with the invention, the first upstream emission control, which receives the exhaust gas generated by the first cylinder group, does not release stored oxygen because the cylinders of the first cylinder group are not operated with an air-fuel mixture rich of stoichiometry. As a result, the invention improves overall vehicle fuel economy because only the second upstream emission control device, which receives the exhaust gas generated by the second cylinder group, is purged of stored oxygen during the purge event.